The present invention relates to polymeric opacifying agents used in the production of paints, paper coatings, and films and, more particularly, to opacifying agents having a core/sheath structure.
The steady increase in the price of titanium dioxide (TiO.sub.2) as well as the steady decrease in its availability have created a demand for products capable of replacing at least a part of the TiO.sub.2 needed to opacify paints and other coating compositions. In TiO.sub.2 and other naturally white products, such as snow, a multiplicity of irregular surfaces and/or microvoids randomly scatter light within these products to create the appearance of whiteness. Microvoid-containing polymer particles perform in a similar manner. When incorporated into a paint or paper coating composition, the microvoid-containing polymer particles cause a significant increase in the resulting composition's whiteness or opacity.
Recently, a number of approaches to incorporating microvoid-containing polymer particles into paint and paper coating formulations has been disclosed. (See "Air and Polymer Extended Paints," Chalmers et al., European Supplement to Polymer Paint Colour Journal, Oct. 5, 1983, pp. 94-102.) One approach entails the use of solid and/or vesiculated (hollow) polyester/styrene copolymer beads having a size in the range of 3 to 50 microns and marketed under the tradename "Spindrift." These particles are formulated to contain both air and TiO.sub.2 within their interior. A major use of these particles has been to formulate matte paints with good polish resistance. However, unpigmented beads act as windows in the film and are generally used only for highly saturated colors. For cost effectiveness, the vesiculated beads have been generally used as a partial replacement for TiO.sub.2.
In a second approach, aggregates of fine particles are formed by a process of addition polymerization under conditions of high shear in an aqueous medium. This process produces irregular shaped particles (sold under the name "Microblock"). The irregular shaped particles allegedly produce higher film strengths than produced by spherical beads. Unlike the vesiculated beads, described above, internal pigmentation with TiO.sub.2 has not been found to be effective. Accordingly, the irregular shaped particles are usually combined with external TiO.sub.2 and a film extender to produce high pigment value content (pvc) paints which are very flat.
In a third approach, fine particle size, non-film-forming, polystyrene hollow beads are used. These beads have a particle size range of the same order of magnitude as that of latex particles normally used in emulsion paints, namely 100 to 600 nm. Typically, the beads are blended with the latex particles in a 1:1 by weight ratio. These particles are sold under the tradename of "Plastic Pigment."
A fourth approach involves a suspension of non-film-forming hollow styrene/acrylic copolymer beads (typically 38% by weight, 52% by volume). The processes for manufacturing these microvoid particles are described in U.S. Pat. No. 4,427,836 (Kowalski et al. I), U.S. Pat. No. 4,469,825 (Kowalski et al. II) and U.S. Pat. No. 4,594,363 (Blankenship et al.). Kowalski et al. I and Blankenship et al. disclose the production and use of water insoluble particles made by sequential emulsion polymerization to form a swellable "core" of polymeric acid. The core is then at least partially encased with a "sheath" or "shell" polymer permeable to a volatile base such as ammonia or an organic amine. Sheath permeability is necessary to allow the base to contact the core. Kowalski et al. II differs from Kowalski et al. I and Blankenship et al. in that in Kowalski et al. II the core comprises a polymeric base and the sheath polymer is permeable to an acid, e.g., acetic acid. When any of these particles are neutralized, the core swells and causes the entire particle to swell up. When a paint film containing these particles dries, water is lost from the beads' interior voids and replaced by air. The resulting empty "microvoids" act as light scattering sources, thus increasing the hiding power of the dried film. In addition to opacifying by scattering incident light, it has been postulated that these particles also contribute to hiding power by uniformly spacing TiO.sub.2 particles and, thus, help prevent TiO.sub.2 clumping. Accordingly, these particles supplement or replace a part of the pigmentary medium or extenders that would otherwise be required to achieve equivalent hiding power. For example, literature articles concerning these products indicate that these particles can replace up to about 25% of the TiO.sub.2 content in a paint formulation. In particular, four parts by volume of these opaque polymers are approximately equal in hiding power to one part by volume of TiO.sub.2.